Quantizers of complex vectors find numerous applications in communication systems. A typical example is a multiple-input, multiple-output (MIMO) communication system. In a typical MIMO communication system, a multi-antenna array in a base station sends multiple data streams selectively and simultaneously to autonomous single-antenna terminals, also referred to as “users,” thereby achieving throughput gains relative to a set of single-antenna links. Multi-user systems of this type are sometimes referred to as MIMO broadcast systems.
One drawback of multi-user MIMO systems is that the base station has to know the propagation characteristics of the forward channel. The process through which the base station obtains this information is generally referred to as training. See, for example, U.S. Patent Application Publication No. 2005/0265290 to Hochwald et al. entitled “Feedback Method for Channel State Information of a Wireless Link,” which is commonly assigned herewith and incorporated by reference herein.
Each of the single-antenna terminals may generate forward channel state information in the form of a corresponding channel vector which characterizes the channel between the base station and that terminal. The channel vectors may be based on measurements made by the terminals using pilot signals transmitted by the base station over the forward channel. Each terminal actually identifies a code vector (or codeword) from a predefined codebook that maximizes a product of the code vector and the channel vector which characterizes the channel between the base station and that terminal. The codeword is thus considered a quantization (approximation) of the channel vector. The terminals transmit their identified code vectors back to the base station over the reverse channel.
The base station uses the identities of the code vectors received from the terminals to precode data in order to improve MIMO data transmissions. However, when components of the code vectors are complex, such measurements require extensive receiver overhead in hardware, software, and/or processing time.
Accordingly, a need exists for improved techniques for quantizing complex vectors in communication systems such as MIMO broadcast systems.